# year    value 
 1901 20.63178 
 1902 20.51799 
 1903 19.94969 
 1904 19.73684 
 1905  19.9945 
 1906 20.66029 
 1907 20.20846 
 1908 20.42454 
 1909 20.17935 
 1910 19.93524 
 1911 19.94874 
 1912 20.46573 
 1913 20.80061 
 1914 20.80058 
 1915 20.47433 
 1916 20.60584 
 1917 20.24735 
 1918 20.58212 
 1919 20.75503 
 1920 20.64704 
 1921 20.41874 
 1922 20.10039 
 1923 20.45634 
 1924 19.68399 
 1925  20.0537 
 1926 20.68668 
 1927 20.25049 
 1928  19.6616 
 1929 20.52738 
 1930 20.49325 
 1931 20.17914 
 1932 20.81401 
 1933  20.0734 
 1934 20.33141 
 1935 19.94881 
 1936 20.03748 
 1937 20.11594 
 1938  19.9997 
 1939 20.92008 
 1940 19.97669 
 1941 19.87351 
 1942  20.4579 
 1943 20.81162 
 1944 20.42505 
 1945 20.44243 
 1946 20.27075 
 1947 20.38495 
 1948 20.10349 
 1949 20.16187 
 1950 20.37674 
 1951 20.37623 
 1952 20.64543 
 1953 20.32085 
 1954 20.27787 
 1955 20.81951 
 1956 19.96306 
 1957 20.24023 
 1958 20.39174 
 1959 20.22676 
 1960 20.68759 
 1961 20.74603 
 1962 20.49366 
 1963 20.59786 
 1964 20.16986 
 1965 20.35189 
 1966   20.187 
 1967  20.0885 
 1968 20.97631 
 1969 20.53825 
 1970 20.71058 
 1971 20.04661 
 1972 20.42407 
 1973 20.98198 
 1974 20.31791 
 1975 20.35126 
 1976 20.54432 
 1977 20.57609 
 1978 20.81785 
 1979 21.22066 
 1980 20.83661 
 1981  21.1904 
 1982 20.85501 
 1983 21.20668 
 1984 20.82156 
 1985 20.51882 
 1986 20.42029 
 1987 20.66899 
 1988  20.6502 
 1989 21.23165 
 1990 21.02869 
 1991 20.96016 
 1992 20.16544 
 1993 20.54667 
 1994 20.35747 
 1995 21.03923 
 1996 20.49794 
 1997 20.91342 
 1998 20.59978 
 1999 21.20316 
 2000 20.68052 
 2001 21.51268 
 2002 20.14735 
 2003 21.12799 
 2004 21.03289 
 2005   20.927 
 2006 21.04604 
 2007 21.14251 
 2008 21.05634 
 2009 21.37527 
 2010 21.13305 
 2011   21.151 
 2012 21.22242 
 2013 21.43975 
 2014 21.55798 
 2015 21.15906 
 2016 21.64073 
 2017 21.53574 
 2018  21.7701